The present invention is directed to rechargeable sodium-ion batteries. In particular, the invention relates to a cathode active material having a high energy density for a sodium-ion battery.
Energy conversion and storage have become more and more important in transportation, commercial and residential applications. In particular, large-scale implementation of renewable energy and the next generation of electric vehicles require development of inexpensive and efficient energy storage systems.
Lithium-ion battery technologies have been dominating the market of portable electronic devices since its first commercial use in 1991. The technology associated with the construction and composition of the lithium ion battery has been the subject of investigation and improvement and has matured to an extent where a state of art lithium ion battery is reported to have up to 200 Wh/L of energy density. However, even the most advanced lithium ion battery technology is not considered to be viable as a power source capable to meet the demands for a commercial electric vehicle (EV) in the future. For example, for a 300 mile range EV to have a power train equivalent to current conventional internal combustion engine vehicles, an EV battery pack having an energy density of approximately 500 Wh/L is required. As this energy density is close to the theoretical limit of a lithium ion active material, technologies which can offer battery systems of higher energy density are under investigation. In addition, the use of lithium-ion batteries in large scale applications is also challenged by the increasingly high cost of lithium and the potential supply risk. Therefore, low-cost, high-capacity, and safe alternatives that are not resource-limited are of particular interest in this field.
Sodium, whose intercalation chemistry is similar to lithium and which is an element with lower cost and higher abundance than lithium, has gained much interest recently. The concept of a battery based on sodium is similar to that of a lithium-ion battery in that a sodium-ion battery also involves the energy carried by the reversible transport of an ion (Na+) between the positive and negative electrodes. However, the reaction mechanism in a sodium-ion cell may be significantly different from the mechanism in a lithium-ion cell. Indeed, the real performance of a specific material in a sodium-ion cell may be distinct from its performance in a lithium counterpart.
In comparison, sodium is less reducing than lithium (−2.71V vs. S.H.E., compared to −3.04V) and the gravimetric capacity is lower (1165 mAh g−1 compared to 3829 mAh g−1) (Johnson et al. Adv. Funct. Mater. 2013, 23, 947-958). The cation radius of sodium is nearly 40% larger than lithium (1.06 Å vs. 0.76 Å) and therefore, any successful intercalation host materials will possess channels and interstitial sites compatible with the larger size of the sodium cation.
For instance, graphite, the commercial anode active material in lithium-ion batteries, cannot accommodate the insertion of Na to a concentration higher than Na0.0625C6 and is electrochemically irreversible. As a result, graphite is unsuitable as a sodium-ion battery anode. Thus, the fundamental differences between lithium and sodium appear to dictate that it may be impossible to simply adopt the knowledge and the techniques developed for lithium-ion batteries to sodium-ion batteries. Appropriate electrode and electrolyte materials that can be used in practical sodium electrochemical cell systems need to be developed for sodium-ion batteries. One barrier to significant progress in the development of Na ion batteries is the identification and development of promising cathode candidates to date.
In ongoing studies of energy generation and storage systems, the present inventors have investigated a wide range of materials for suitability as cathode active agents in a sodium battery. Phosphate based materials have been considered as excellent cathode candidates because of their high stability and low cost. However, most phosphate cathodes show poor electronic conductivity and as a result, full capacity of the cathode can't be achieved with general charge/discharge processes. To overcome the conductivity problem, conductive material coating technique and nanosizing the cathode material have been utilized.
Another approach to obtain a cathode of high capacity is to employ a transition metal capable of multiple electron transfer and thus able to assume more than one sodium. Vanadium is well-known to be capable of transfer of two electrons, such as from the +5 to +3 oxidation state. Thus the inventors have considered that Vanadyl phosphate (VOPO4) is a material combining the merits of vanadium and of phosphate and theoretically has the possibility to show high capacity as well as good stability as a cathode active material for a sodium battery.
Vanadium phosphate materials have been described as cathode materials.
For example, U.S. Pat. No. 6,872,492 (Barker et al.) describes sodium ion batteries based on cathode materials of the general formula: AaMb(XY4)cZd. Example 4b describes synthesis of VOPO4xH2O and Examples 4c and 4d describe synthesis of NaVO PO4. Charge and discharge of a cell containing a cathode of the NaVO PO4 and a negative electrode of lithium metal is described. Sodium ion cells prepared are based on a carbon composite negative electrode and NaVO PO4F as the positive electrode active material.
U.S. 2013/0034780 (Muldoon et al.) describes a magnesium battery and lists VO PO4 as a suitable positive electrode active material.
U.S. 2004/0048157 (Neudecker et al.) describes a lithium solid state thin film battery containing a lithiated vanadium oxide film as an anode and as one possible cathode material, LiVO PO4.
U.S. 2013/0260228 (Sano et al.) describes a lithium secondary battery having as a positive electrode material, a compound of the formula: Lia(M)b(PO4)cFd. LiVO PO4 is described in a preferred embodiment.
U.S. 2013/0115521 (Doe et al.) describes a magnesium secondary battery wherein the current collectors are coated with a thin protective coating. VO PO4 is listed as a positive electrode active material.
U.S. 2012/0302697 (Wang et al.) describes a magnesium cell having a carbon or other graphitic material as a cathode active material. VO PO4 is included in a list of other cathode active materials.
However, none of these references disclose or suggest ε-vanadyl phosphate as a high capacity cathode active material for a rechargeable sodium battery.
It is an object of this invention to provide safe, efficient and a high capacity cathode active material for use in a sodium battery.